It is often the case that transformers are used in data communications transceivers to interface with telecommunications networks. Such transformers are often used to both transmit and receive data signals. Unfortunately, transformers can often cause distortion to such data signals. In particular, transformers will often create harmonics of the data signals and other anomalies.
The creation of distortion by transformers is undesirable because it may interfere with the data signals and cause errors. For example, a received data signal may often have a diminished magnitude and power due to significant attenuation across a relatively long local loop which electrically couples a data communications transceiver to a central office. The distortion caused by a transformer may be of sufficient magnitude to interfere with such diminished data signals, resulting in data errors.
As a consequence, it is desirable to measure the distortion of a transformer accurately to determine the acceptability of a transformer for data communications applications. However, accurate measurement of the distortion caused by transformers presents obstacles. For example, harmonics created by a transformer that cause signal degradation may be up to 90 dB lower than the magnitude of a signal transmitted into the transformer. In addition, noise and distortion existing in typical test signals applied to a transformer are indistinguishable from the signal degradation and noise caused by the tested transformer. As a consequence, it is often necessary that measuring equipment have a significant dynamic range greater or equal to 90 dB to measure the harmonic distortion relative to the transmitted data signal. Generally, measuring equipment with such a dynamic range tends to be quite expensive.
Proposed solutions include the use of various filters to eliminate unwanted noise. For example, noise and distortion created by the test signal source may be filtered out using a band pass filter tuned to the center frequency of the test signal before the signal is passed to a transformer to be tested. In addition, a notch filter may be used to eliminate the sinusoidal test signal allowing only distortion components to pass to an analyzer.
Unfortunately, the proposed solutions have proven to be inadequate. In particular, generally high quality filters are required which increase cost. Further, a unique filter is employed for each test signal frequency used, further driving up cost. In addition, such filters create additional electrical load that should be accounted for in the circuit.
Consequently, there is a need for a testing circuit that reduces the magnitude of a transmitted signal received from a transformer without affecting the harmonics created by the transformer in order to determine the distortion caused by a transformer used in data communications applications.